Chemical and mineral compositions of sediments from ODP Site 127-797

In order to reconstruct past variations in the aeolian dust (Kosa) contribution to the Japan Sea, and to establish a direct link between terrestrial and marine climatic records, we have applied statistical procedures to distinguish and quantify detrital subcomponents within the detrital fraction of the late Quaternary hemipelagic sediments in the Japan Sea. Q-mode factor analysis with varimax and oblique rotation of the factors followed by multiple-regression analysis between mineral composition and factor loadings was conducted using six ''detrital'' elements. Four detrital subcomponents were defined, which are attributed to Kosa derived from ''typical'' loess, Kosa from ''weathered'' loess, and fine and coarse arc-derived detritus, respectively, based on comparisons with the chemical and mineral compositions of probable source materials. Using these detrital subcomponents, the variation in Kosa fraction was reconstructed for the last 200 ky. The results reveal millennial-scale as well as glacial-interglacial scale variations in Kosa contribution. Especially, millennial-scale variability of Kosa contribution suggests the presence of high frequency variation in summer monsoon precipitation in the central to east Asia during the last 200 ky.

(Table 1) Bulk and clay mineralogy at DSDP Leg 66 Holes

During Leg 66 eight sites on an active margin transect off southern Mexico were drilled in order to determine the nature of ocean-continent transition across a subduction zone. Present outcrops of Mesozoic to Precambrian basement at the coast intruded by Mesozoic magmas within only 65 km of the Middle America Trench axis indicate truncation of the continental margin, tectonic removal of an accretionary zone, and consumption of ocean sediments and crust by subduction.

Lithologic, mineral, chemical and isotopic compositions of altered rocks with increasing distance across the eastern flank of the Juan de Fuca Ridge, ODP Leg 168 data

During ODP Leg 168, 10 sites were drilled across the eastern flank of the Juan de Fuca Ridge (JdFR), to examine the conditions of fluid-rock interaction in three distinct hydrothermal regimes (referred to as the Hydrothermal Transition (HT), Buried Basement (BB) and Rough Basement (RB) transects), extending over a ~120 km linear transect perpendicular to the spreading ridge. This was carried out in an attempt to constrain the conditions and processes that control the location, style and magnitude of low temperature (<150°C) fluid-rock interaction within this setting. This paper presents new data on the petrology, mineral chemistry and whole rock strontium and oxygen isotopic compositions of basalts from the eastern flank of the JdFR, in order to investigate the extent, style and sequence of low-temperature hydrothermal alteration and to establish how the hydrothermal regime evolved with time. Throughout the flank, a progressive sequence of low-temperature hydrothermal alteration has been identified, marked by changes in the dominant secondary mineral assemblage, changing from: chlorite+chlorite/smectite; to iron oyxhydroxide+celadonite; to saponite+/-pyrite; culminating at present with Ca- to CaMg(+/-Fe,Mn)-carbonate. The changes in secondary mineralogy have been used to infer a series of systematic shifts in the conditions of alteration that occurred as the basement moved off-axis and was progressively buried by sediment. In general, hydrothermal alteration of the uppermost oceanic crust commenced under open, oxidative conditions, with interaction between unmodified to slightly modified seawater and basaltic crust, to a regime in which circulation of a strongly modified seawater-derived fluid was more restricted, and alteration occurred under non-oxidative conditions. Across the flank, petrological observations and microprobe analyses indicate that the observed ranges in secondary mineral composition are directly related to changes in the geochemical and textural characteristics of the basement, as well as to interaction between fluids and phases from the four stages of alteration. This is suggestive of an increase in fluid-rock increased with time. Whole rock 87Sr/86Sr and d18O analyses of basalts from across the eastern flank of the JdFR reinforce petrological observations, with 87Sr/86Sr and d18O values slightly elevated above accepted pristine MORB values for this region. These results are consistent with an increase in the amount of fluid-rock interaction with time. Across the flank, enrichment in the 87Sr/86Sr and d18O relative to MORB, is influenced by a number of factors, including: local and regional variations in the crustal lithology and structure; the age of the crust; the extent of bulk rock alteration; and theoretically, the relative abundance of different isotopically-enriched secondary mineral phases in the crust.